1. Field of the Invention
The present invention relates to a shape measuring method and a shape measuring device, including a contact probe that measures with high precision (that is, on the order of nanometers), the shape of a surface of an optical element (such as a lens or a mirror), and the shape of a surface of a die for producing the optical element. The present invention is applicable to, in particular, a shape measuring device that can measure a shape having a steep inclination angle, such as a wall having an angle of 90 degrees.
2. Description of the Related Art
In general, a contact-type shape measuring method and a contact-type shape measuring device, used to measure the shape of, for example, a lens or a mirror, have the following features.
FIG. 8 shows a first related example disclosed in Japanese Patent Registration No. 03272952. In FIG. 8, a contact probe 28 is supported by, for example, a leaf spring 27 so as to be movable in an axial direction with respect to a probe holding unit 26, and is controlled so that its displacement is constant. The position and posture of the probe holding unit 28 at this time are measured with a laser length measuring device 10, 11, 14 (also called an interferometer), and data of the position and data of the posture are calculated, to determine the displacement of the contact probe 28.
FIG. 9 shows a second related example disclosed in Japanese Patent Registration No. 03063290. In FIG. 9, a displacement sensor 10 that measures a displacement in an axial direction of a contact probe 2, supported so as to be movable in the axial direction, and that measures a displacement in a direction perpendicular to the axial direction is provided, to measure and correct the inclination of the contact probe 2.
FIGS. 10A and 10B show a third related example disclosed in Japanese Patent Laid-Open No. 2005-037197. In FIGS. 10A and 10B, a displacement sensor 5 that measures a displacement in a direction perpendicular to an axial direction of a contact probe 2 is provided. In addition, Japanese Patent Laid-Open No. 2005-037197 discusses a unit that estimates a horizontal contact force generated at a surface of a measurement object due to contact with the contact probe 2, a unit that detects a vertical contact force generated at the surface of the object due to contact with the contact probe 2, and a unit that detects inclination angle information at each measurement position of the surface of the measurement object. Further, the document discusses controlling contact force in the axial direction so that a vertical dragging force generated at the surface of the measurement object when a following operation is performed is constant.
Such shape measuring devices may be called a contour measuring device in other documents. The contact probes may be called a sensing pin, a sensing pin member, or a feeler in other documents, but will be called a probe in the specification. A laser length measuring device is a device that measures a length on the order of nanometers, and is also called an interferometer from its measurement principle. In the specification, the laser length measuring device is primarily called an interferometer. The term “contact force” may be used to mean a vertical dragging force.
The above-described related arts have the following problems.
(1) Difficulty in Stabilizing a Scanning Speed of the Contact Probe at a Steeply Inclined Surface
The above-described related examples 1, 2, and 3 discuss methods of scanning a measurement surface while controlling the contact probe in the vertical direction. That is, a feed-back controlling operation using a contact force that the contact probe receives is carried out only in the vertical direction. However, since a scanning direction of the contact probe at an inclined surface always includes a vertical component, a scanning speed is always influenced by a control deviation of a following control operation of the contact probe. This influence becomes larger as the inclination angle becomes steeper, thereby making it difficult to stabilize the scanning speed of the contact probe. For a vertical surface having an inclination angle θ of 90 degrees, the control deviation of a controlling system that tries to make a contact force constant causes the scanning speed to change. As a result, the problems that a measurement precision is reduced and that the scanning speed of the probe cannot be increased occur. That is, it is difficult for the probe to scan a steeply inclined surface.
(2) Difficulty in Controlling Contact Force of Contact Probe at Steeply Inclined Surface
The difficulty will be described with reference to FIG. 10B used in the aforementioned Japanese Patent Laid-Open No. 2005-037197. FIG. 10B shows the contact probe 2 in contact with a measurement object having an inclination angle of θ. Here, the vertical contact force is Nz, the horizontal contact force is Nx, and a contact force in a direction of a normal line to a surface of the measurement object is Nn. In the related example 1, the contact probe 28 is controlled in the vertical direction so that the vertical contact force Nz in FIG. 10B is constant. In the related example 3, the contact probe 2 is controlled in the vertical direction so that the contact force Nn in the direction of the normal line to the surface of the measurement object is constant.
However, as the inclination angle θ becomes steeper, the amounts of change of Nz and Nn with respect to an amount of vertical movement of the contact probe are reduced. Therefore, it becomes difficult to control the probe so that the contact probe 2 follows a steeply inclined surface of the measurement surface, thereby resulting in the problem that a measurement precision is reduced and in the problem that the scanning speed cannot be increased. In particular, for a vertical surface having an inclination angle θ of 90 degrees, even if the contact probe 2 is moved vertically, the vertical contact force Nz and the contact force Nn in the direction of the normal line to the measurement surface 8 do not change. Therefore, the contact probe 2 is incapable of being controlled so that the contact probe 2 follows the measurement surface.
In the above-described related example 1, the controlling of the probe 28 is carried out with a Z slide 4. Since a movable area that covers an entire measurement area is required for the Z slide 4, the device becomes structurally large. Therefore, a high rigidity cannot be provided, and natural frequency is reduced. These effects appear within a control loop that makes the contact forces constant. Consequently, control gain cannot be made high. As a result, the frequency that allows the probe to follow the measurement surface is limited to a low value, thereby preventing the scanning speed from being increased.